The invention relates first of all to a drive device for a movable part in an advance stroke, a power stroke and a return stroke, especially a mold closure device for an injection molding machine.
As a mold closure device for an injection molding machine, the drive device moves the movable die platen of the machine. Such a drive device has to satisfy two important requirements. First, it must move the mold platen as fast as possible to close and open the mold, so that the cycle time for the production of a molding can be kept short. Secondly, it must be able to hold the mold platen and hence the entire mold closed with great force against the high injection pressure. First, then, adjustment movements must be performed with high acceleration, high speed and powerful braking; secondly, high forces must be exerted without any substantial movement. Such requirements may arise not only with the closure unit but also with the ejectors or the injection unit of a plastics injection molding machine. For example, when plastic is injected into the mold, the plasticizing screw has to be moved toward the mold at relatively high speed until the mold is completely filled with plastic. If subsequently the plastic contained in the mold is exposed to what is referred to as holding pressure, the drive has to apply a high force without substantial movement of the plasticizing screw.
A mold closure device in which an attempt was made to satisfy the above-mentioned requirements is disclosed in DE 195 23 420. In this drive device, there is connected to the movable mold platen the piston part of a hydraulic cylinder-and-piston unit having a hydraulic piston, which borders with a first active surface on a first pressure chamber and with a second active surface, which is smaller direction thereto, on a second pressure chamber, and comprises a further hydraulic piston, which is fixedly connected to the first hydraulic piston, and possesses a third active surface, which borders on a third pressure chamber and is active in the same direction as the second active surface. Via a valve arrangement, the second pressure chamber can be connected to the first pressure chamber and can be relieved of pressure separately from the first pressure chamber. The further hydraulic piston is formed by a plunger-like projection on the side of the first hydraulic piston facing the second pressure chamber.
During the adjustment movement in the closing direction, the first pressure chamber and the second pressure chamber are connected to one another, so that only the quantity of pressure medium determined by the difference between the first active surface and the second active surface has to flow to the cylinder-and-piston unit and a high speed can be achieved. For the application of the locking force, the second and the third pressure chambers are relieved of pressure, so that the first active surface is fully available to apply the locking force. During the adjustment movement in the opening direction, in the conventional mold closure device, all three pressure chambers are connected to one another and to a source of pressure medium, so that the quantity of pressure medium to be fed to the cylinder-and-piston unit for a desired speed is determined by the cross-sectional surface area of a piston rod starting from the first hydraulic piston, emerging from the cylinder after traversing the first pressure chamber and fixed to the movable part.
In the conventional mold closure device, the cylinder-and-piston unit for closing and opening the mold is operated in open hydraulic circuits, the pressure chambers being connected to a tank at the end of the closure operation and at the end of the opening operation and the movable part evidently being braked only by friction.
It is an object of the invention further to develop a drive device of the conventional type so that it can be operated at higher dynamics.
This object is achieved with a drive device according to the invention. In this drive device, a first, receiving hydraulic cylinder-and-piston unit has a first pressure chamber, which can be subjected to the action of pressure in the power stroke alone, and a third pressure chamber to be subjected to the action of pressure on the return stroke. A second, delivering cylinder-and-piston unit has a fourth pressure chamber which is connected to the third pressure chamber at least on the advance stroke and on the return stroke, and a fifth pressure chamber which can be connected to the first pressure chamber. By a drive motor, in particular an electric drive motor, the piston part and cylinder of the second cylinder-and-piston unit are movable relative to one another. A second pressure chamber of the drive device is located in one of the two cylinder-and-piston units. A valve arrangement is present via which, during the advance stroke, pressure medium can be forced from the second pressure chamber into the first pressure chamber and, during the power stroke, the second pressure chamber can be relieved of pressure separately from the first pressure chamber. In addition, the third pressure chamber and the fourth pressure chamber have cross sections such that pressure medium forced out of the third pressure chamber during the advance stroke can be received by the fourth pressure chamber. The fourth pressure chamber may be permanently connected to the third pressure chamber. However, the fourth pressure chamber may also be formed by two partial pressure chambers, of which, during a working step to tear open the mold, only one partial pressure chamber is connected to the third pressure chamber. In this manner, the active size of the fourth active surface is diminished. A stronger force transmission can be achieved.
The method according to the invention for operating a drive device according to the invention is one characterized in that, during the movement of the movable part, all five pressure chambers are subjected to the action of an elevated pressure greater than atmospheric pressure. Preferably, the pressure during the braking of the movable part remains above atmospheric pressure in each of the five pressure chambers, so that the braking force is particularly high and the braking travel correspondingly short.
Advantageous embodiments of a drive device according to the invention are listed below.
The second pressure chamber of a drive device according to the invention may be located on the second cylinder-and-piston unit.
However, particular preference is given to an embodiment in which the first hydraulic cylinder-and-piston unit comprises a first hydraulic piston, which borders with a first active surface on the first pressure chamber and with a second active surface, which is smaller than the first active surface and faces in the opposite direction thereto, on the second pressure chamber, and comprises a further hydraulic piston, which is fixedly connected to the first hydraulic piston, and possesses a third active surface, which borders on the third pressure chamber and is active in the same direction as the second active surface. The second hydraulic cylinder-and-piston unit comprises a second hydraulic piston, which is connected to a fourth active surface at the fourth pressure chamber, which is permanently connected to the third pressure chamber, and borders with a fifth active surface, which faces in the opposite direction to the fourth active surface, on the fifth pressure chamber. In addition, the dimensional relationship between the fifth active surface and the difference between the first and second active surfaces is equal to the dimensional relationship between the fourth active surface and the third active surface.
Particularly preferably, the further hydraulic piston is formed in a simple manner by a plunger-like projection on the side of the first hydraulic piston or second hydraulic piston facing the second pressure chamber.
Advantageously, in a drive device according to the invention, the first cylinder-and-piston unit is formed in the same way as that according to DE 195 23420 C1.
If either the third pressure chamber or the fourth pressure chamber comprises a first partial pressure chamber and a second partial pressure chamber, of which one partial pressure chamber is connected jointly with the other partial pressure chamber via a valve to the other pressure chamber and can be connected separately from the partial pressure chamber connected to the other pressure chamber to a low-pressure tank, a force transmission to tear open the mold of a plastic injection molding machine is possible.
Preferably, the fourth pressure chamber and the third pressure chamber can be connected via a valve to a low-pressure tank. This reliably prevents the formation of a vacuum in the third and in the fourth pressure chambers when a high pressure is built up in the first pressure chamber by movement of the second hydraulic piston, during which time the first hydraulic piston and the further hydraulic piston move only slightly, if at all. The risk of cavitation is then slight. The low-pressure tank is expediently a closed tank with no connection to atmosphere and with a volume compensation provided by a flexible wall or a gas cushion. In this manner, the hydraulic system of the drive device can be so formed that the pressure medium has no contact with atmosphere. If the pressure medium is oil, this slows the process of aging. If the pressure medium is water, this results in no oxygen being introduced from the atmosphere so that corrosion of the metal parts that come into contact with the water is minimized.
It is advantageous if the fifth active surface on the second hydraulic piston is smaller than the first active surface on the first hydraulic piston. The second hydraulic piston and the first hydraulic piston then form a hydraulic force transmitter, so that with a relatively low load on the drive motor and on a rotation/translation converter installed downstream thereof a high force can be exerted with the first hydraulic piston. The force transmission naturally corresponds to a reduction of travel.
The drive device can be operated at particularly high dynamics if it is operated with elevated prestress pressures in the pressure chambers. In order to permit such operation in an advantageous manner, the fifth pressure chamber on the second hydraulic piston is connected in a first position of a further valve to the first pressure chamber and in a second position of the further valve to the low-pressure tank. In a xe2x80x9cprestressxe2x80x9d working step before the opening of the mold, it is now possible, by moving the second hydraulic piston in the xe2x80x9cmold openingxe2x80x9d direction, for pressure medium to flow from the low-pressure tank into the fifth pressure chamber, so that after the further valve has been switched over and the fifth pressure chamber connected to the first pressure chamber, the level of the prestress pressures is largely maintained. The level falls only slightly, because the pressure medium present in the fifth pressure chamber is raised to the pressure level of the first pressure chamber.